The present invention relates to an alumina sintered body on which a heater electrode is printed so as to form a ceramic heater or sensor element heater.
An alumina sintered body generally contains low-sodium aluminate as a main material and a sintering assistant or grain growth-inhibitor for lowering the sintering temperature. Exemplary sintering assistants include Na.sub.2 O.sub.SiO.sub.2 -MgO glass components and oxides which easily form vitreous structures when being sintered such as MgO, CaO, K.sub.2 O, and B.sub.2 O.sub.3. When the oxide or oxides are added to low-sodium aluminate, vitreous structures with a low melting point are formed in the alumina sintered body, which results in lowering the sintering temperature.
Addition of an IIIa metal oxide such as Y.sub.2 O.sub.3 to low-sodium aluminate also lowers the sintering temperature of the alumina sintered body, and addition of MgO or ZrO.sub.2 inhibits grain growth in the alumina sintered body. An oxide or oxides like Y.sub.2 O.sub.3, MgO, and ZrO.sub.2 may thus be added to low-sodium aluminate so as to sufficiently lower the sintering temperature.
Some problems arise when an alumina sintered body containing low-sodium aluminate and the above oxide or oxides is used as a heater base, however. When a heater including an alumina sintered body as a heater base is used at a high temperature and a high voltage of direct current, alkaline metals and alkaline earth metals existing in the alumina as impurities migrate to the negative terminal side and are segregated as compounds with low melting points, which results in lifting of the surface of the heater base. This can cause cracks on the heater base or breaking of heater wires.
More specifically, in a heater including a heater base of an alumina sintered body containing SiO.sub.2, the following are observed: destruction of a grain boundary in the heater base due to sublimation of SiO.sub.2, occurrence of chipping on the heater base, and lowering of strength of the heater base as SiO.sub.2 forms vitreous structures with a low melting point when the heater is used at a high temperature. One of the primary reasons for these phenomena is that SiO.sub.2 and Al.sub.2 O.sub.3 combine to form mullite, which decomposes, melts, and separates into two phases, i.e., a silica phase and an alumina phase at high temperatures.
Migration of alkaline metals and alkaline earth metals may be prevented by using high purity Al.sub.2 O.sub.3 for the heater base. Use of high purity of Al.sub.2 O.sub.3, however, raises the sintering temperature to approximately 1,560.degree. C. When sintering is executed at such high temperatures, grain growth takes place in a heater electrode including Pt, or only a relatively rough sintered body is obtained, which results in shortening the durability of the heater. Thus, use of high purity alumina is not a fundamental solution since it raises the sintering temperature to be much higher than the optimum sintering temperature, i.e., 1,520.degree. C. The optimum sintering temperature, 1,520.degree. C., is determined by considering the fact that grain growth of Pt or other elements takes place in the heater electrode at a temperature higher than 1,550.degree. C.
Wherefore, the object of the invention is thus to provide an alumina sintered body which has long durability and is sintered at 1,520.degree. C. or a lower temperature.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.